Here's how to install the Add-In after downloading it from ArcGIS Marketplace
Browse to the folder location of the EarthCache Add-in file (*.esriAddInX) that you downloaded on your computer.
Double-click the file to open the Esri ArcGIS Add-In Installation Utility window.
To continue, click Install Add-In. The Add-In is copied to the default location and installed. A copy of the file remains in the location where you browsed to it.
If ArcGIS Pro is open, exit and restart the application. The Add-In is now available for use.
Download WebView2 if using Add-In for ArcGIS Pro 2.9
If the Add-In is being installed for ArcGIS Pro 2.9, a download of WebView2 is required. You will see a message when logging in for the first time to download WebView2. Visit the URL to start the download, or click here to download WebView2. Note: Visiting this URL will immediately start download in your browser. If you're unable to download WebView2, see instructions on how to get around this requirement.